The objective of this work is to understand the basis of membrane electrical events in heart muscle and their role in imitating or regulating contraction. This involves the ionic basis of the action potential and the role of passive membrane properties in excitation and conduction. Studies in excitation-contraction coupling involve control of membrane potential by voltage clamp and tension recording. Ionic currents recorded in the voltage clamp are used both for studies of membrane behavior and for control of contraction. Ion-specific microelectrodes are used to determine internal and external activities of Na and K. During this grant period specific goals include 1) modelling of the Purkinje fiber voltage clamp and steps to improve the technique, 2) studies into the chloride current and the second inward current, 3) measurements of intracellular Na activity related to inotropic state, and 4) definition of reasons for low intracellular ion activity coefficients.